All the head shapes are sharing the same rig now, and it appears that male/female shapes are indeed compatible with all the races, which will save on a lot of extra shape keys.
For various reasons I decided to revert to my original plan of making the base mesh gender and race neutral, which meant figuring out a workflow for averaging all the shape keys into the base mesh without upsetting everything else. Consequently as more shapes are added the base should become increasingly “average”. Also note that the male shape key works as the inverse of the female one (and vice versa).
Just about had a schizoid embolism trying to nut out the formula for all this…